hurd: handling pending signals could result in corruption of FPU state

hurd: handling pending signals could result in corruption of FPU state

Handling a pending signal calls _hurd_setup_sighandler() once again
after the initial signal handling. In this case a pointer to the
previous sigcontext is available to supply the interrupted thread's
original basic state, fpu state and fpu XSTATE. The original XSTATE
was not being preserved by the pending signal but instead overwritten
with the active XSTATE. XSTATE register values modified by the
signal handling code could therefore be wrongly propogated back to
the interrupted user code.